The drag experienced by a vehicle moving through air at motorway speeds (or “freeway” speeds, in American English) is the single largest contributor to that vehicle's fuel consumption. If the drag force on a vehicle can be reduced, less fuel will be required to run the vehicle, reducing running costs and environmental damage caused by the burning of fuel.
Drag on a vehicle from the air in which it moves is made up chiefly of two components: friction drag and pressure drag. FIG. 1 shows a large goods vehicle (LGV) 10 in motion. The vehicle comprises a cab 11 and a trailer 12. The cab 11 and trailer 12 together form a bluff body. A bluff body is a body for which the boundary layer separates from much of the surface of the body. It is therefore a feature of bluff bodies that the pressure drag on the body while it is moving through a fluid is a much greater proportion of the total drag than is the frictional drag. Thus the pressure drag on the LGV while it is moving through air accounts for a significant proportion of the total drag; for an articulated lorry (or “truck”, in American English), pressure drag can be as much as 90% of the total drag.
The pressure drag on a body is proportional to the pressure at the front of the body moving into the air minus the pressure at the opposite end of the body. The pressure drag on the LGV 10 shown in FIG. 1 is proportional to the pressure at the front of the cab 11 minus the pressure at the back of the trailer 12. There are, therefore, three approaches to reducing the pressure drag on this vehicle: reducing the pressure at the front of the vehicle, increasing the pressure at the back of the vehicle, or a combination of the two.
Attempts have been made to increase the pressure at the back of an LGV by introducing fairings on the vehicle to streamline it. However, when these fairings are arranged at the back of the vehicle, they increase the length of the vehicle and can therefore reduce its manoeuvrability and can also interfere with vehicle loading.
An object of at least certain embodiments is to address one or more of these problems.